1. FIELD OF THE INVENTION
The present invention relates to a thread cutting device in a stitch forming machine which forms a chain of stitches on a fabric by using a lower thread supplied from a bobbin holder positioned below a needle hole of a throat plate and an upper thread supplied from a needle reciprocally vertically moving toward and against the bobbin holder through the needle hole.
2. DESCRIPTION OF THE RELATED ART
One of the conventional thread cutting devices is shown in FIG. 17 which includes a fixed knife C disposed below and adjacent a needle hole B of a throat plate A and a movable knife D for cutting a thread E located between a bobbin holder (not shown) and the needle hole B.
The thread cutting device of this type is particularly applied to a multi-color embroidery machine which is normally provided with a plurality of needle bars. Through each needle bar, an upper thread of different color is threaded and a chain of stitches of different color may be formed by selectively reciprocating one of the needle bars. Normally in this embroidery machine, only the upper thread is cut when the color is to be changed in the case that a pattern with multiple color is embroidered, while both the upper thread and the lower thread are cut when the whole pattern has been embroidered.
Means for selectively effecting the cutting only of the upper thread and the cutting of both the upper thread and the lower thread is disclosed in Japanese Laid-Open Pat. Publication Nos. 63-9487 and 63- 115592. These publications propose the provision of a lower thread sweeping lever which moves across the area below the needle hole for sweeping the lower thread to the position where the movable knife does not act on it, and an upper thread engaging lever which separates the upper needle thread and the upper fabric thread from each other and moves the upper needle thread to the area where the movable knife does not act on it, so that only the upper fabric thread (including the lower thread if it is not swept by the lower thread sweeping lever) may be cut by the movable knife.
In place of using the upper thread engaging lever, it is also proposed that the movable knife itself interposes between the upper fabric thread (including the lower thread if it is not swept by the lower thread sweeping lever) and the upper needle thread so as to separate them from each other prior to reaching the cutting position.
Thus, the conventional thread cutting device is constructed essentially to cut both the upper fabric thread and the lower thread, and the lower thread is moved away when it is not desired to be cut.
However, as described above, in this kind of the switch forming machine, only the upper fabric thread is cut when the color is to be changed in the case that a pattern with multiple color is embroidered, while both the upper fabric thread and the lower thread are cut when a whole pattern has been embroidered. Therefore, the frequency of cutting of only the upper fabric thread is higher than that of the cutting of both the upper fabric thread and the lower thread. Thus, the conventional thread cutting device is disadvantageous in that the lower thread sweeping lever must be frequently moved.
Further, although the lower thread sweeping lever moves across the area below the needle hole, it must not interfere with the upper needle thread which also passes through the needle hole. Therefore, the conventional thread cutting device is further disadvantageous in that the space for mounting the lower thread sweeping lever is very limited and that the lower thread sweeping lever must be moved in a large stroke, so that the thread cutting device must be of large size.
Returning to FIG. 17, in the conventional thread cutting device, the fixed knife C includes at the bottom thereof a cutting edge C1 of substantially right angle. The cutting edge C1 is positioned at one end of the bottom opposite to the direction toward the cutting position of the movable knife D indicated by an arrow. The movable knife D includes at one end thereof toward the cutting position an engaging portion D1 for engagement with the thread E and includes at an opposite end a cutting edge D2. As the movable knife moves toward the cutting position, the thread E extending downwardly toward the bobbin holder is engaged by the engaging portion D1 and is thereafter cut by the cooperation of the cutting edge D2 of the movable knife D with the cutting edge C1 of the fixed knife C. The lower portion E1 of the cut thread E is hold between the bottom of the movable knife D and a leaf spring F for a subsequent stitch forming operation.
In this thread cutting device, as the movable knife D moves in the direction indicated by the arrow, the engaging portion D1 firstly goes under the fixed knife C, so that the thread E between the engaging portion D1 and the needle hole B contacts the cutting edge C1 of the fixed knife C. Therefore, if the tension applied to the thread E becomes larger (particularly in the case that thread E is a lower thread), the thread E may be accidentally worn out at the cutting edge C1 and cut before the cutting edge D2 of the movable knife D cooperates with the cutting edge C1 of the fixed knife C. Thus, the length of the cut lower portion E1 becomes shorter to the extent that it cannot be held by the leaf spring F, causing difficulties in the subsequent stitch forming operation.